1. Field of Invention
The present invention relates to pulverizers and mixers. Specifically, the present invention relates to crushers, grinders and mixers of the type designed to process coal, biomass material, and other materials.
2. Description of Related Art
The need for renewable energy sources and the creation of equipment capable of producing a marketable fuel has been increasing dramatically. In the last few years, local, state, and federal regulators have made two primary changes in the laws affecting energy producers using renewable sources. First, tougher clean air standards under the federal Clean Air Act and state laws restrict the type of materials a fuel can emit when burned. Second, the federal government has deregulated the ways in which power may be marketed. This deregulation offers energy producers greater incentive to maximize their power output within the emission limits.
Not surprisingly, current research and development for many different fuel types has focused on methods and products which would enable producers to increase energy output without exceeding present environmental standards. One fuel alternative, which has been found to meet environmental standards, mixes coal with wood or other biomass materials to create a hybrid fuel. Current equipment for commingling materials (i.e., crushers, grinders, and mixers) is generally not considered effective due to several problems in the breaking down of the biomass material: inability of such equipment to handle different material types, improper mixing techniques, inability to produce a product whose particle's size has a distribution that is advantageous for combustion, and the unacceptably high amounts of energy consumed in preparing the fuel. If such problems were overcome, biomass fuel, such as wood, would be a viable alternative capable of increasing power production under the current clean air standards.
Although crushers, grinders and mixers have been around for over a century, these types of devices are unable to grind biomass material finely enough to be used in power plants. To solve this problem, conventional reduction systems often require the material to pass through several stages to reach its final size as a result of the size limitations of the crushing machines and their internal parts. Such solutions add substantial expense to fuel preparation, and yield the array of problems listed previously.
One type of crusher and grinder design provides a chamber with pivoting arms mounted on a shaft. The arms accelerate material into the machine wall, the collision with which breaks the material. Another type of crusher or grinder uses pivoting hammers on a first shaft, which usually intermesh with hammers of a second shaft, to break the material by slamming into it. See U.S. Pat. Nos. 629,262, 4,082,231, and 4,973,005. Both designs are inefficient as a result of the significant wear on internal parts of the machine. This wear makes the machines prone to breaking and maintenance and results in significant downtime for parts replacement. Furthermore, wear causes losses in machine efficiency because devices having worn parts consume more power to perform their functions. Interdigitating designs especially suffer excessive wear because material is crushed between the meshing arms. In addition, machines relying on physical contact with machine parts to reduce the size of the material produce particles of uneven size that have sharp edges. These types of design also increase the temperature of the material significantly because the collisions with machine parts create friction. In addition, in order for these machine to maintain a certain capacity, the exit temperature of the material must be over one hundred and fifty degrees Fahrenheit. This exit temperature is too high for certain low combustion temperature materials.
Other pulverizing designs rely on cyclonic turbulence to reduce the size of material. Cyclonic turbulence may be created by the rotation of two shafts in the same direction to produce two fluid streams traveling in opposite directions in between the two shafts. The opposite forces acting on the material located in between the shafts causes the material to collide with each other and consequently break. Some designs using cyclonic turbulence also rely on the material's colliding with the parts of the machine and like material in order to complete the reduction. See U.S. Pat. Nos. 410,247, 430,646, and 1,457,693. These designs, however, do not effectively use all of the force created through the inertia of particle collision. Conventional devices experience a loss in force created at the intersecting point of the two material streams because the material does not intersect directly head-on, but rather at a seventy to eighty degree angle. The most effective collision occurs when two materials streams collide at a one hundred and eighty degree angle, i.e., a head-on collision.
U.S. Pat. No. 5,400,977 discloses a pulverizing system in which drill cuttings are broken down by colliding with each other, but not through cyclonic motion. In this device, pivoting, intermeshing arms throw material into collision with material thrown by other arms. The arms are housed within a tank whose top includes two semi-circular portions through which the arms carry the material as they rotate. The collisions of material occur below the intersection of the two semi-circular portions and between the intermeshing arms. This arrangement does not maximize the amount of inertia created by the rotating arms and therefore, is not an efficient method of reducing material. This arrangement loses inertia because the collisions are not head-on, as a result of the configuration of the tank, and because the pivoting arms decelerate when they encounter the material. Furthermore, as discussed above, the intermeshing arms suffer excessive wear because some of the material is crushed between them.